Composting methods

Three basic methods of composting—windrow, static pile, and in-vessel—are described below.

(i) Windrow method—The windrow method involves the arrangement of compost mix in long, narrow piles or windrows (fig. 10-30). To maintain an aerobic condition, the compost mixture must be periodically turned. This exposes the decomposing material to the air and keeps temperatures from getting too high (>170 °F). The minimum turning frequency varies from 2 to 10 days, depending on the type of mix, volume, and the ambient air temperature. As the compost ages, the frequency of turning can be reduced.

The width and depth of the windrows are limited only by the type of turning equipment used. Turning equipment can range from a front-end loader to a automatic mechanical turner. Windrows generally are 4 to 6 feet deep and 6 to 10 feet wide.

Figure 10-30 Windrow schematic

Concave to Normal

Figure 10-30 Windrow schematic

Concave to Normal

Some advantages and disadvantages of the windrow method include:

Advantages:

• Rapid drying with elevated temperatures

• Drier product, resulting in easier product handling

• Ability to handle high volumes of material

• Good product stabilization

• Low capital investment

Disadvantages:

• Not space efficient

• High operational costs

• Piles should be turned to maintain aerobic conditions

• Turning equipment may be required

• Vulnerable to climate changes

• Odors released on turning of compost

• Large volume of bulking agent might be required

(ii) Static pile method—The static pile method consists of mixing the compost material and then stacking the mix on perforated plastic pipe or tubing through which air is drawn or forced. Forcing air through the compost pile may not be necessary with small compost piles that are highly porous or with a mix that is stacked in layers with highly porous material. The exterior of the pile generally is insulated with finished compost or other material. In nonlayered operations, the materials to be composted must be thoroughly blended before pile placement.

The dimensions of the static pile are limited by the amount of aeration that can be supplied by the blowers and the stacking characteristics of the waste. The compost mixture height generally ranges from 8 to 15 feet, and the width is usually twice the depth. Individual piles generally are spaced about a half the distance of the height.

With forced air systems, air movement through the pile occurs by suction (vacuum) or by positive pressure (forced) through perforated pipes or tubing. A filter pile or material is normally used to absorb odor if air is sucked through the pile (fig. 10-31).

Some advantages and disadvantages of the static pile method include:

Advantages:

• Low capital cost

• High degree of pathogen destruction

• Good odor control

• Good product stabilization

Disadvantages:

• Not space efficient

• Vulnerable to climate impacts

• Difficult to work around perforated pipe unless recessed

• Operating cost and maintenance on blowers

Figure lO-Sl Static pile composting schematic

Figure lO-Sl Static pile composting schematic

(iii) In-vessel method— The in-vessel method involves the mixing of manure or other organic waste with a bulking agent in a reactor, building, container, or vessel (fig. 10-32) and may involve the addition of a controlled amount of air over a specific detention time. This method has the potential to provide a high level of process control because moisture, aeration, and temperature can be maintained with some of the more sophisticated units. Dead animal composting in a composting bin as discussed in section 651.1007(b), Dead animal disposal, is an example of unsophisticated in-vessel composting.

Some of the advantages and disadvantages of the in-vessel method include: